1. Technical Field
The present invention relates generally to electronic filters used in the cable television industry, and relates more particularly to the construction and assembly of such filters.
2. Background Art
Typical electronic filter constructions in the cable television (CATV) industry involve a considerable number of parts, such as, for example, one or more circuit boards, connecting wires or leads, filter circuit components, isolation plates, blocks or chambers, input and output terminals, moisture barrier seals or plugs, connector housings, sub-housings or caps, o-rings, outer housing sleeves, and potting material. This elaborate array of parts constrains efforts to: minimize the size and weight of the filters; reduce material and labor costs associated with assembly of the filters; and simplify and automate the assembly process. Examples of such filter constructions are shown and described in: U.S. Pat. No. 5,278,525 to Palinkas; U.S. Pat. No. 4,901,043 to Palinkas; U.S. Pat. No. 4,701,726 to Holdsworth; U.S. Pat. No. 4,451,803 to Holdsworth et al; U.S. Pat. No. 3,579,156 to Parfitt; and U.S. Pat. No. 3,065,434 to Calderhead.
For such CATV filters as highpass and lowpass filters, diplex filters, windowed highpass filters, and step attenuator (or return path) filters, tunable filter circuits and shielding between filter components and circuits are not normally required. Thus, for these types of filters, an opportunity is presented to simplify filter components, construction and assembly. U.S. Pat. No. 5,745,838 to Tresness and Zelenz (Zelenz is a named inventor of the present invention) discloses (FIGS. 8-10) a filter construction for a return path filter called a "step attenuator." This construction is also shown in FIG. 2 herein, as representing the prior art construction for this type of filter. While simplification was achieved in U.S. Pat. No. 5,745,838, the construction still required two major o-rings around the male and female terminal caps and an outer housing sleeve (See FIG. 2 herein); and, manual assembly of these parts was still required.
Many diplex, windowed highpass, and return path filters (See, e.g., embodiments shown in FIGS. 1-5 of U.S. Pat. No. 5,745,838), have dual (or "parallel") circuit paths. For example, FIG. 1, herein, shows a simplified step attenuator circuit 10 containing a forward (or highpass) path 12 and a return (or lowpass) path 14. Cascaded or elongated circuit board arrangements such as shown in U.S. Pat. No. 5,770,983 to Zennamo, Jr. et al., U.S. Pat. No. 4,901,043 to Palinkas, U.S. Pat. No. 4,701,726 to Holdsworth, U.S. Pat. No. 4,451,803 to Holdsworth et al., U.S. Pat. No. 3,579,156 to Parfitt, and U.S. Pat. No. 3,065,434 to Calderhead, are not optimum platforms for such dual path filters. A more optimum platform would be to have two circuit boards disposed in a parallel arrangement.
U.S. Pat. No. 5,278,525 to Palinkas discloses parallel circuit boards for a CATV notch filter (or "trap"), rather than for a dual path filter. The construction includes a considerable number of extra parts, such as an isolation shield, circuit board housings, tuning screw housings, o-rings, and an outer housing sleeve.
In most CATV applications, the filters are installed in an outdoor environment. Thus, it is important that the filter construction be moisture resistant. Efforts to make filters moisture resistant have included enclosing the filter in an outer housing sleeve and employing o-rings between the filter and the outer housing sleeve. See, e.g., U.S. Pat. No. 5,745,838 to Tresness et al., U.S. Pat. No. 5,278,525 to Palinkas, U.S. Pat. No. 4,701,726 to Holdsworth, and U.S. Pat. No. 4,451,803 to Holdsworth et al. Such an approach requires the additional parts and expense of o-rings and outer housing sleeves, and may require manual assembly of such parts.
A prime path for moisture penetration into the filter is through the terminal fittings or connectors. While efforts to prevent moisture penetration through filter connectors (such as disclosed in U.S. Pat. No. 5,278,525 to Palinkas) have been satisfactory, there remains a need to improve moisture resistance through these connector paths.
Another consideration in CATV filter construction is to establish a good and reliable electrical ground between the filter circuit or circuits and the filter housing. Electrical ground has been established by soldering or fitting isolation shields or blocks between the circuit boards and the filter housing, or by soldering wires or leads between the circuit board and housing. See, for example, U.S. Pat. No. 4,701,726 to Holdsworth. However, such methods usually require additional components or manual assembly steps.